The present invention relates to a butterfly valve suitably used for changing flow paths for hot fluid, for example, an automobile exhaust gas.
A butterfly valve has been used for changing flow paths for hot fluid such as a hot combustion gas, for example, an automobile exhaust gas. The butterfly valve sometimes has a problem of difficulty in opening and shutting. This is because 1) seizure in a bearing portion, 2) adhesion of oxidized scales peeling off a pipe, and/or 3) an increase of a coefficient of friction at a high temperature between a material for a duct and a material for a flap cause(s) an increase of a torque required.
There have been other problems, i.e., deterioration of sealability when the flap is shut because of thermal deformation of the flap or corrosion and/or abrasion of the contact portion of the flap and an outer cylinder, leakage of a hot gas outside, and an increase of rotational resistance because of corrosion or abrasion of a shaft-bearing portion.
Therefore, an object of the present invention is to provide a butterfly valve which smoothly opens and shuts without changing the driving force for opening and shutting even in a hot-gas flowing path for a long period of time even in the case of an adhesion of oxidized scales or an increase of a coefficient of friction at high temperatures and which is free from thermal deformation of a flap and gas leakage by corrosion of a flap, an outer cylinder, a bearing portion, shaft, or the like.